This still is very limited in what it can lerp, but it sets the stage for arbitrary lerps later.

Documentation changes, and, in particular, the ability to lerp from
and to any decoration by lerping via null.

Reorder the code, remove some redundant documentation, improve the
interpolation logic.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

* Revert "Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)"

This reverts commit ceb814aa.

Revert "Change all ocurrences of '$runtimeType#$hashCode' to use the idAndType method. (#10871)" (#10880)

This reverts commit d46e208b.

* Change all instances of '$runtimeType#$hashCode' to use the describeIdentity method.

The describeIdentity method generates a shorter description with a consistent length
consisting of the runtime type and the a 5 hex character long truncated
version of the hash code.

...and rearrange a bunch of code so that all these arguments/members
are always in the same order.

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

Image listeners installed in paint handlers need to know whether the listener
is being called during the paint.

Fixes https://github.com/flutter/flutter/issues/4937

Previously we lerped them backwards.



Fixes #2832